1) The study was initiated to determine the structure and molecular weight of the genomic RNA of the avian myelocytoma virus MC29. Visualization of the RNA was achieved by using a modified Kleinschmidt-technique permitting optimal extension of the molecules. The presence of two subunits of different sizes was established. 2) The study was initiated to characterize the morphological aspects of the virus-host cell relationship of new isolates of endogenous viruses of the Southeast Asian mouse. A novel association of intracytoplasmic viral precursors of type A with the mitotic apparatus, especially the microtubules, was discovered. By using microtubule inhibitors, the association of viral precursors with the microtubule system of the cell was confirmed.